Electrography

ABSTRACT

An electrographic apparatus and process for producing, on a record medium having a layer of insulating material in contact with an electrically conductive backing member, an electrostatic charge image corresponding to an image to be recorded comprises an image grid and at least one control grid arranged between a corona discharge device and the layer of insulating material. The image grid comprises an electrically conductive core having insulating and conducting areas defining the image to be produced. The control grid is electrically conductive and arranged in spaced and generally parallel relation to the image grid and between the latter and the corona discharge device. When the core of the image grid and control grid are individually biased to a potential for establishing electrical fields of different strengths between the respective areas and the backing member and a flow of ions is directed toward the grids and the record medium, the flow of ions through the grids is modulated by the electrical fields to produce an electrostatic charge image on the layer of insulating material.

United States Patent Frank [4 Aug. 1, 1972 [54] ELECTROGRAPHY [57]ABSTRACT Inventor! Lee FitZPatl'ick Frank, Rochester An electrographicapparatus and process for producing, on a record medium having a layerof insulating [73] Assignee: Eastman Kodak Company, material in contactwith an electrically conductive Rochester y backing member, anelectrostatic charge image corresponding to an image to be recordedcomprises an [22] Filed: sept- 1965 image grid and at least one controlgrid arranged [21] AWL Nod 492,988 between a corona discharge device andthe layer of insulating material. The image grid comprises an electri-Related U.S. Application Data cally conductive core having insulatingand conduct- [63] Continuation-in-part of Set. No. 452 095 April ing limage Pmduced- The 30 1965 abandoned trol grid is electricallyconductive and arranged in spaced and generally parallel relation to theimage grid and between the latter and the corona discharge 8|. device.when the core of the image grid and control [58] Field d i s e d i c li"555/5"; "I51 grid are individually biased to a potential forestablishing electrical fields of different strengths between therespective areas and the backing member and a flow of ions is directedtoward the grids and the record [56] References Cited medium, the flowof ions through the grids is modu- UNITED STATES PATENTS lated by theelectrical fields to produce an electro- 3 220 324 1 H1965 Snemng 355/16static charge image on the layer of insulating material. 3:393:6177/1968 Gaynor .355/3 13 Claims, 60 Drawing Figures PATENTEDAus 1 m23.680.954

, BY /%w ATTORNEYS PATENTED 1 I973 3.680.954

sum 05 0F 10 ,g 904 F/G' 29 938 3 902 CIA (g @lil 904 LEE F- FRANK l 902INVENTOR.

r 4,7 I] (a (Nil /Q J-z FIG- 28 929 ATTORNEYS multiple copying ofdocuments.

With reference to the document copying embodiment of this invention,there are at present two types of commercial electrophotographicdocument copying processes. One employs a single-use copy paper having aphotoconductive coating and the other employs a reusable drum having aphotoconductive coating. Both processes employ the image-wise dischargeof a uniformly charged photoconductive insulating layer by image-wiseexposure to produce an electrostatic charge image. This charge image isdeveloped to a visible image by depositing thereon a finely dividedpowder or toner. The resultant powder image may be fixed to thephotoconductive layer (as is done in the process which employs asingle-use copy paper) or it may be transferred to another surface (asis done in the process which employs a reusable drum).

The electrophotographic process employing the reusable drum has thefollowing disadvantages which are overcome by the present invention. Themachine is very expensive for the low-volume user; it does not do a goodjob of copying photographs and large solid areas; the machine isrelatively complex and requires more than the usual amount ofmaintenance; it uses a selenium coated drum which is expensive andfragile and which must be replaced periodically (normally, after aboutevery 40,000 to 50,000 copies); the process requires a developed-imagetransfer step; and it has a relatively low sensitometric speed. Theelectrophotographic process which employs a single-use,photoconductor-coated, copy paper has the following disadvantages whichare overcome by the present invention. Copies must be made on paperhaving a coating of photoconductive insulating material; because of thiscoating, the paper is relatively expensive; the paper is heavier thanordinary paper; copies can be marred by scratching with metals; theprocess has a relatively low sensitometric speed; and the transparency,luster, color, dullness, etc. of such paper are to a great extentdependent upon the appearance of the photoconductor.

With reference to the document copying in color embodiment of thisinvention, conventional electrophotographic systems, if used for colorprinting would involve either (1) the transfer of developed images whichtransfer has inherently associated therewith problems of registration or(2) repeated development on top of the same sensitive surface, with theproblems of strong inter-image effects. In addition, in the latter casethere would be difficulty in obtaining good whites due to the dyesensitizer in the photoconductive coating.

With reference to the duplicating embodiment of the invention,conventional duplicating systems either require large, expensivemachines and long makeready times or have the problems of poor quality,limited numbers of copies, and of being messy.

With reference to the character printing embodiment of the invention,conventional printers involve the use of mechanical elements to make animpression on the page. These elements consequently become worn and arelimited in writing rate by inertia. Xerographic and photographiccharacter printers have a somewhat greater printing rate than mechanicalprinters but need a special cathode ray tube and the xerographic onesare relatively insensitive. Photographic recording requires a relativelylong processing time, and does not provide an inexpensive, real-size(standard type size), quick-access copy. The size of the type in thephotographic copy is usually substandard.

With reference to the multiple copying of documents, this embodimentincorporates the above discussed advantages of both the document copyingand the duplicating embodiments of the invention. The subject embodimentcan be used to make either a single copy or a very large number ofcopies from a single exposure.

It is an object of the present invention to provide an electrographicrecording system.

It is a primary object of the present invention to provide anelectrophotographic document copying and duplicating system which isfree of all of the abovementioned disadvantages of the present systems.

It is thus an object of the present invention to provide anelectrophotographic copying system which is inexpensive, capable ofcopying onto a large variety of materials, employs an indefinitelyreusable photoconductive insulating member, does not require thedeposition of toner onto a photoconductive insulating member, and whichcan be embodied in a small, light and inexpensive machine.

It is a further object of the invention to provide an electrographicsystem which, in addition to overcoming all of the above disadvantagesof the prior systems, exhibits very high sensitometric speeds andprovides extreme flexibility.

It is a still further object of the invention to provide anelectrographic color recording system which eliminates theabove-mentioned problems of prior color systems such as the requirementfor transfer with its associated registration problems and therequirement for repeated development on top of the same sensitivesurface with the associated strong interimage effects.

It is a further object of the invention to provide an electrographiccolor recording system which is fast, which allows dyes to be chosenprimarily for stability and color and not for their chemical properties,which does not require charge transfer or toner transfer, which providesfor adjustment of color balance on a single print rather than only on aseries of prints therefore reducing the rejection rate, which allows theuse of white light in inspecting the process, which provides forvariable contrast by electronic controls and by design of thephotoconductive element, which employs low cost equipment, whichprovides an inherently correct neutral scale rendition, which is subjectto color balancing, in which sufficient, deliberately introducedinterimage effects are available to accomplish color masking, and whichprovides greatly decreased color degradation (improved color rendition)compared to color xerography because the sensitive surface is above thepreviously deposited toner and is not affected by it 1 therebyeliminating autopositive interimage effects.

It is a still further object of the invention to provide anelectrographic duplicating system in which the printing-master can bemade in any of a number of different ways, in which the printing-masteris simple, inexpensive and rugged, and in which the duplicating processcan be carried out using the same apparatus used for document copying bysimply replacing the photoconductive element with the printing-master.

It is another object of the invention to provide an electrographiccharacter printing system which overcomes the above-mentioneddisadvantages of prior systems.

It is another object of the invention to provide an electrographicalphanumeric character printing system which involves no moving parts inthe printing head, which has high speed and durability, which allows themaking of quick-access, stable, real-size, black-onwhite prints, inwhich a wide range of type sizes can be obtained, which allows selectionof arbitrary type fonts, which uses less expensive copy paper than thatwhich the photographic systems use, which provides real-time writingcapability for a computer thus eliminating or at least reducing the needfor a buffer or printer allocating stage, which has a wide range ofapplications, such as ticker-tape, radio or wired-teletype, andpunched-card printer, etc., and which eliminates the need for anexpensive character display cathode ray tube.

It is another object of the invention to provide a process and apparatusfor making multiple prints from a single exposure.

These objects are accomplished by the following invention. I havediscovered a unique electrographic recording system which comprisesdirecting a fiow of ions toward a record medium and imagewise modulatingthe flow of ions to produce an image on said record medium. Theimagewise modulation is accomplished by interposing a grid or an arrayof grids in the flow of 10118.

The primary difference between the several embodiments of the inventionis in the nature of the grid or grids used. In the document copyingembodiment of the invention, the grid is a photoconductive grid. Thecolor recording embodiment uses the same photoconductive grid butemploys color separation filters in the exposing step and differentlycolored developers in the developing step from the document copyingembodiment. The duplicating embodiment employs a grid similar inappearance to the photoconductive grid but different in construction inthat it employs an imagewise distributed coating of insulating materialon a conductive grid. One character printing embodiment employsconductive grids formed in the shape of the characters to be printed.The multiple copying of documents embodiment employs bothphotoconductive grids and insulator coated grids.

In the preferred embodiments of the invention a record medium is usedwhich has an insulating surface coating on a relatively conductingsupport layer. The flow of ions, as imagewise modulated by the grid orgrid array, produces an electrostatic charge image on the insulatingsurface, which charge image can be xerographically developed to producea visible image.

In the document copying embodiment of the invention, for example, theimagewise modulation of the flow of ions is accomplished by means of aphotoconductive grid comprising a biased or grounded electricallyconductive core or grid which is, in the preferred embodiments, coveredwith a layer of photoconductive insulating material.

It has been found extremely important for the operation of thisinvention that the photoconductor completely cover all of the exposedsurface of the conductive grid with a uniform coating. Even microscopiccracks or holes in the photoconductive coating on the grid aredetrimental to the operation of the process.

This photoconductive grid is positioned directly in the ion flow andpreferably just above the record medium. The photoconductive grid isimagewise exposed to produce a conductivity image in the photoconductivematerial, while the flow of ions is being directed through the grid andtoward the record medium and, hence, can be designated as an image gridmeans. The terms electrically energize and imagewise energize areintended to encompass, for the purpose of this specification and claims,the biased or grounded, electrically conductive areas of the grid. Inthe areas of the grid which are insulating or non-energized (where thegrid is not exposed) the flow of ions will first produce a small surfacepotential (from a few to a few hundred volts) on the grid surface andwill then pass through the grid to charge the underlying insulatingsurface of the record medium. In the conducting, and thus imagewiseenergized, areas of the grid (where it is exposed) the ions are capturedby the grid and are thus removed from the ion flow, whereby the areas ofthe record medium underlying the exposed or energized areas of the gridremain uncharged. An electrostatic charge image corresponding to thelight image is thus formed on the insulating surface of the recordmedium. This electrostatic image is then xerographically developed andthe developed image fixed to the record medium or alternativelytransferred to a final receiving sheet, in which case the record mediumcan be cleaned and reused.

One of the most startling efiects noted with this invention is inconnection with the exceptional sensitivity of the process. Inxerography, because the process is electrostatic, the maximum operatinggain of the system is unity. The operating gain of the system is definedas the number of stored charges removed by one absorbed photon. In thepresent system, which is electrodynamic, operating gains greater thanunity can be achieved. This process has exhibited speeds of up to 300times the speed of present electrophotographic systems. An additionallystartling effect in this connection is that of an increase in effectivespeed with increased resolution, i.e., a ZOO-line per inchphotoconductive grid gives many times the speed of a l00-line per inchgrid. A 300-line per inch grid gives speeds over times faster thanprevious electrophotographic systems and previously consideredunavailable. This simultaneous increase of speed and resolution isopposite to the relationship found in other systems, for example, inphotography.

A great degree of flexibility is available in designing anelectrophotographic document copying machine to operate on theprinciples of the present invention, as

will be evidenced by the following discussion. Many kinds ofphotoconductors (both N and P types) having various levels ofsensitivity and dark current, can be used. Useful photoconductors are,among others, cadmium sulfide, selenium, selenium and telluriummixtures, zinc oxide, arsenic trisulfide, cadmium telluride, cadmiumselenide, germanium (PN or NP) and organic photoconductors such astriphenylamine in an insulating organic resin vehicle (such as that soldunder the trademark Vitel PE-lOl) sensitized with 2,4-bis-(4-ethoxyphenyl)-6-(4-n-amyloxy-styryl) pyrylium fiuoroborate. Further itis possible to vary the effective sensitivity of a given photoconductorwithin the system. For example, electrical bias can be used to employphotoconductors that do not exhibit low resistance upon exposure;increasing the corona current changes the time constant depending on thegeometry of the source and the nature of the photoconductor; andincreased exposure tends to decrease time constants. It is understood ofcourse that other materials which exhibit a change in conductivity uponactivation can be used in the present invention in place of aphotoconductor. Such other materials include photoinsulators, i.e.,materials which are normally conductive but which become insulating uponexposure to light, and heatsensitive materials which exhibit a change inconductivity when heated. Hence, the image grid means can be consideredas being coated with a radiation responsive insulating material. A layerof photoconductive insulating material may be formed on the grid, forexample, by evaporation techniques or by spray coating. It is necessary,however, to spray or evaporate from a widely diverse number of angleswith respect to the grid so that all of the surface of the grid will becompletely covered with a uniform layer of photoconductor, including theinside walls of the holes in the grid. Woven mesh is hardly suitable forthis process, if coated by evaporation, particularly in the finerweaves, because of the difficulty of completely covering the wiresurface in the region where the wires cross each other. Spray coatingcan be used to totally coat woven mesh for this process, provided thecoating has reasonable leveling or wetting action on the mesh. A widechoice is also available in connection with the electrically conductivegrid which forms the core of the photoconductive grid and to which thephotoconductive material is applied. The choice as to the shape, size,material and method of manufacture is large. It has been found that anetched or electroforrned mesh is superior in mechanical properties forone-to-one document copying; window screening works well for moderatesize posters and is stronger and less expensive; and hardware cloth canbe used for very large prints. Neither the toner nor any part of thedocument copying apparatus ever needs to come into contact with thegrid; the grid is thus indefinitely reusable. It is also relativelysimple to construct and relatively inexpensive. An extremely wide choiceof record mediais available, including ordinary paper at low humidities.The record medium can be, for example, a single layer of insulatingmaterial, a sheet of paper or other support having a thin insulatingcoating, a sheet of thermal-deformable plastic or an ion-sensitivesilver halide emulsion layer, but is preferably an insulating surfacewith a relatively large capacitance per unit area. The record medium,after formation of the electrostatic image thereon, can bexerographically developed, for example, by any of the well-known methodsand the developed image can be fixed thereon to form the final copy, orthe toner can be transferred to a final copy sheet and fixed thereto, inwhich case the record medium can be cleaned and reused. In this lattercase, the record medium can conveniently be in the shape of a rotatabledrum; in connection with this embodiment it should be noted that thedrum does not have an expensive, fragile, photosensitive coating butrather a simple, rugged, inexpensive, electrically insulating coating.Although the preferred embodiments of the invention utilize a recordmedium having an insulating surface whereby an electrostatic image isproduced thereon which can be xerographically developed, it is notedthat the invention is not limited to such record media. For example, aBerchtold layer (a mosaic of conducting areas separated by insulatinglayers as described in US. Pat. No. 2,866,903) can be positioned behindthe photoconductive grid with an insulating recording sheet positionedbehind the Berchtold layer. Further, the record medium can be aconducting sheet which changes color in response to a flow of currenttherethrough, as is known in photoconductography. If the record mediumis replaced with an electroluminescent panel, the electrographic systemfunctions as a light amplifier. Since the electroluminescent panel glowswhere there is current, the image has a tonal scale reversed from thatof the normal (i.e., a negative). When the gain is less than unity, thereversed tonal scale would be useful either in direct viewing of aphotographic negative as a positive, or in converting a negative to apositive. In other words, the imagewise pattern of ions coming from thegrid can be used to form a record (either permanent or temporary) invarious ways.

Further, a wide choice of exposing methods is available, includingprojection and contact exposing methods with either area exposure orline scanning. The associated benefits of line scanning are usable, i.e.rightreading, wrong-reading and coand counter-current scanning. Variousscanning embodiments useful in the invention include (I) stationarycorona charger, grid and lens with moving document and record medium forboth coand counter-current scanning, and (2) stationary document andrecord medium with (a) moving corona charger, grid and lens system (b)moving corona charger with stationary grid and lens system, (c) movingcorona charger and lens system with sta tionary grid, and (d) movingcorona charger and grid with stationary lens system. The conductivityimage produced on the photoconductive grid may thus be spatial (in thecase of large area exposure) or temporal (in the case of line scanning)or both in the case of small area scanning.

Many types of ion sources, including corona discharge electrodes such asneedles and wires, are well-known in the art and any of such may be usedin the present invention.

Certain arrangements of the document copying embodiment of the inventionrequire the use of a photoconductive material which will remainconductive (exhibit persistence of conductivity) for a period of timeafter the illumination has been turned off and in the presence of acorona discharge. The persistence of conductivity of a photoconductivematerial is often altered in the presence of a corona discharge. Forexample, in zinc oxide photoconductive layers, the photoconductivitywhich would normally persist for many minutes is destroyed in slightlyover a second in the presence of a corona discharge even at very lowcorona levels. Contrarywise, in certain cadmium sulfide photoconductivelayers, normally the photoconductivity will decay in less than 0.1second, but under a corona will exhibit persistence of conductivity forseveral minutes. i

The above description is applicable to the color recording embodiment ofthe invention since it also employs a photoconductive grid. Many of theunique advantages of this color recording embodiment result directlyfrom the fact that toner is not deposited on the photosensitive layer asit is in the prior systems. Thus there is no need to transfer toner andno associated registration problems. Further, there are no detrimentalinterimage effects caused by charging, exposing, and

developing a photosensitive layer containing previously deposited toner.The dyes to be used can be chosen for their color and stability. Manyother advantages of this embodiment are disclosed above in the objectsof the invention.

The duplicating embodiment employs a grid comprising a grounded orelectrically biased conductive core or grid having an imagewisedistribution of-insulating surface areas. The insulating and conductiveareas of the grid modulate the flow of ions in the same manner asdescribed above with respect to the photoconductive grid. The primarydifferencebetween the two embodiments is that the image is effectivelypermanent (permanent for the duration of the copy run) on theprinting-master grid. As stated above this embodiment has manyadvantages over the known duplicating systems.

The character printing embodiment employs a grid comprising a groundedor biased conductive electrode formed in the shape of a character to beprinted. This grid modulates the flow of ions in somewhat the samemanner as do the grids of the above embodiments. The grid is energizedto either attract or repel the ions in the flow of ions to produce anelectrostatic charge image or shadow of the grid. A stack of individualelectrodes, made of thin wire in the shape of characters, or a gridcomprising a plurality of individual electrode segments corresponding toparts of characters, can be moved across the record medium to reproducea page of type. The corona current is transmitted to the paper only whena character is to be printed. The d.c. corona current can be turned onand off by auxiliary means, the dc. supply, or extra grids. Eachelectrode or electrode segment is connected to ground or to a biaspotential through a switch. The switch (or switches in the case of thegrid employing electrode segments) corresponding to the character to beprinted is closed and the remaining electrodes have practically noblocking effect and do not cast a shadow. Alternative to the abovedescribed scanning motion, a complete row of grids with a full set ofelectrodes for each space in the line can be used to provide printing ofa line as a whole. The switches can be photocells to allow the switchingto be done by light. In one embodiment, described below, the

photocells are arranged in an X-Y array. The light pattern can besimultaneous, as in the case of exposure through a punched card, orsequential, as in the case of the output of a cathode ray tube.

The multiple copying of documents embodiment employs a photoconductivegrid or grid array to produce an electrostatic charge image on aninsulating grid (or on itself or another photoconductive insulating gridin the dark) and then this grid having the electrostatic charge image isused to modulate an ion flow to imagewise charge an insulating recordmedium. Many hundreds of copies can be produced from a single exposure.This embodiment employs a different principle of operation from that ofthe previously described embodiments. In this embodiment ions are notimagewise removed from the ion flow by means of conductive grid areas.The ion flow is modulated by electrostatic fields. The ions areprevented from flowing through the areas of the grid which have thecharge image, but flow freely through the remaining areas of the grid.

in the present specification and claims the term ion flow or flow ofions is employed in describing the step of imagewise charging the recordmember. Although it is true that the preferred source of charges is acorona discharge electrode and the preferred charges are air ions, it isto be recognized that electrons, other charged subatomic particles,charged particles of matter, etc., can be used as the flow of charges inthe present invention which flow is directed toward the record memberand imagewise modulated to produce an electrostatic charge imagethereon. This charge image can be made visible by any known xerographicdeveloping methods. Various types of charges can be used in the presentinvention and it is intended that the term flow of ions and ion flow beinterpreted to include any of such charges and that it not be limited inmeaning strictly to air ions. The grid or conductive mesh in thisinvention is analogous to the grid in a vacuum tube in whichrelationship the term grid is generally defined as an electrode havingone or more openings for the passage of ions therethrough, whichelectrode exercises control on the passage of ions without collectingmore ions than is necessary. Thus, the use of the term grid for theelectrode of the invention which controls the flow of ions to the recordmedium is consistent with present usage of the tenn. The term grid, asused in the present specification and claims is intended to encompassany and all electrode configurations which allow for the passage of ionstherethrough; the term grid thus encompasses such constructions as arealso known by the terms screen, mesh", perforated plate, slot" etc.Since the resolution of theultimate image depends on the number ofopenings per linear inch in the grid and since this is commonly calledlines per inch in halftone production, the same phrase lines per inchwill be used in the present specification and claims to define the sizeof the grid. This term together with information about the percent ofopen area of the grid adequately defines the size of the grid. It isnoted that the resolution of the grid system is determined by the numberof holes per unit length only in the case of stationary operationwithout interaction between the holes. An example of this is the singlegrid system operated stationary, close to the record sheet. If ascanning system is used, the system in the direction of motion has aresolution equal to the reciprocal of the diameter of the holes, or slotwidth, if there is no interaction between the holes and the timefrequency response of the system is not limiting. The relationships inthe scan perpendicular direction are much more complex. There is amarked difference between the stationary and the scanning relationshipsfor cases where only a small portion of the area of the grid is open, orsingle apertures are used. In these cases the slow scanning resolutionis higher than the stationary exposure. The slot is a limiting case inthat the resolution in the scanning direction is obtained only by virtueof the scanning operation. With respect to the embodiments of theinvention which use photoconductors, the phrase imagewise exposing thegrid means, of course, imagewise exposing the photoconductor to suitableradiation to which the photoconductor is sensitive. As used in thespecification and claims, exposing includes exposing to visible light,x-rays, alpha, beta, and gamma rays, and particulate radiation. Any typeof radiation may be employed that will render the photoconductorconductive. In the specification and claims, the term insulating as usedwith respect to certain types of record media is intended to encompassany material which will hold an electrostatic charge image for a periodof time long enough to allow for the development thereof. The period oftime needed to develop an electrostatic image may be extremely short, asin the case of thermal-deformable or electro-deformable plastic sheet.The term potentia or connected to a predetermined potentia is intendedto include any potential including ground potential. The imagewisemodulation of the flow of ions can be either spatial or temporal orboth; line scanning would be both, point scanning would be onlytemporal, and overall exposure would be only spatial.

These and other embodiments of the present invention will be more fullyunderstood by reference to .the following detailed description of theinvention when read in connection with the accompanying drawings, in

which:

FIG. 1 is a schematic illustration of one document copying embodiment ofthe present invention;

FIGS. 2-6 are enlarged, perspective views of various photoconductivegrids useful in the invention;

FIG. 7 is an enlarged cross-sectional view through anotherphotoconductive grid useful in the invention;

FIG. 8 is a greatly enlarged cross-sectional view through aphotoconductive grid and a record medium which illustrates certainprinciples of the invention;

FIGS. 9A and 9B each schematically illustrate a multigrid embodiment ofthe invention;

FIG. 10 is a schematic diagram of an equivalent circuit of the circuitshown in FIG. 1;

FIG. 11 is a schematic diagram of an equivalent circuit of the circuitshown in FIG. 9A;

FIG. 12A is a schematic illustration of a two-grid system;

F IG. 12B is a graph showing the characteristic output current of thecircuit of FIG. 12A;

FIGS. 13-15 each schematically illustrate another multigrid embodimentof the invention;

FIGS. 16-20 each schematically illustrate a document copying embodimentof the invention;

FIG. 21 shows, greatly enlarged, an alternative exposure station for usein the embodiment of FIG. 19;

FIGS. 22A-22C schematically illustrate a simple reflex printingembodiment of the invention;

FIG. 23 is a schematic diagram of a document copying apparatus accordingto one embodiment of the inventron;

FIG. 24 illustrates a lens system useful in the embodiment of FIG. 23;

FIG. 25 is a schematic illustration of an embodiment in which the gridis controlled by a photoconductor spaced from the grid:

FIGS. 26-28 schematically illustrate an embodiment of the inventionwhich employs a photoinsulating material:

FIG. 29 schematically illustrates an embodiment employing a layer ofcellular material on top of the photo conductive grid for the productionof reversals;

FIG. 30 is a schematic illustration of a color reproduction embodimentof the invention;

FIG. 31 is a schematic illustration of a duplicating embodiment of theinvention;

FIG. 32 shows a duplicating embodiment of the invention employingmultiple grids.

FIG. 33 is a schematic illustration of a character printing embodimentof the invention;

FIG. 34 is a plan view showing a line of character printing grids foruse in printing a line at a time;

FIG. 35 is a schematic illustration of a grid composed of electrodesegments for use in printing alphanumeric information;

FIG. 36 is a schematic illustration of a alphanumeric character printingembodiment of the invention;

FIG. 37 is a schematic illustration of a modification of the characterprinting embodiment of FIG. 33;

FIG. 38A schematically illustrates certain principles of operation ofthe embodiment of FIG. 33 when the grid is biased to attract ions;

FIG. 38B is a graph showing the charge density across an electrostaticimage produced by the embodiment of FIG. 38A;

FIG. 38C is a graph showing the nature of the toner deposit on theelectrostatic image of FIG. 38A;

FIG. 39A schematically illustrates certain principles of operation ofthe embodiment of FIG. 28 when the grid is biased to repel ions;

FIG. 39B is a graph showing the charge density across an electrostaticimage produced by the embodiment of FIG. 39A;

FIG. 39C is a graph showing the nature of the toner deposit on theelectrostatic image of FIG. 39A;

FIGS. 40A and 40B schematically illustrate a twostep process 'usingthree grids to produce multiple copies from a single exposure;

FIGS. 41A and 41B schematically illustrate a variation of the embodimentshown in FIGS. 40A and 40B employing an integral array of grids using aforaminous insulating spaces;

FIG. 42 schematically illustrates another integral grid constructionuseful in the process 'of FIGS. 41A and 41B;

FIGS. 43, 44, and 45 schematically illustrate more complex grid arrayscontaining larger numbers of grids for use in the multiple copying ofdocuments embodiment of the invention;

FIGS. 46A, 46B, and 46C show a preferred threestep process for makingmultiple copies from a single exposure;

FIG. 47 schematically illustrates the exposure step of another gridstructure useful for reflex printing is the general process of FIGS.46A, 46B, and 46C; and

FIG. 48 is aschematic illustration of an apparatus employing, forexample, the grid of FIG. 47 for making single or multiple copies ofdocuments by reflex exposure.

I DOCUMENT COPYING FIG. 1 illustrates one embodiment of the presentinvention. In FIG. 1 a transparency 10, having an image to bereproduced, is illuminated by a light source 12. The image is focused bya lens 14 onto a photoconductive grid 16. The grid 16 consists of agrounded, electrically conductive electrode core or grid 18, for exampleof metal, completely covered with a layer 20 of photoconductiveinsulating material. Positioned immediately below the grid 16, is arecord medium 22 consisting of an electrically insulating layer 24 on asupport 26, such as paper. It should be noted that the insulating layer24 is not, or at least need not be, a photoconductive insulatingmateriaLThe record medium 22 can therefore be quite inexpensive. Thesupport 26 is positioned in overlying contact with a grounded fieldelectrode 28 during the step of imagewise charging the insulating layer24. During the imagewise exposure of the grid 16, a corona discharge isproduced adjacent the grid 16 but on the opposite side thereof from therecord medium 22. The corona discharge is produced, for example, byconnecting a corona discharge electrode 32 to one terminal of a voltagesource 34 by means of a switch 36. The other terminal of the voltagesource 34 is connected to ground 38. The

corona discharge provides a source of ions, and the electric fieldproduced between the corona discharge electrode 32 and the fieldelectrode 28, directs a flow of ions toward the record medium 22. In theimagewise exposed areas of the grid 16, the photoconductive layer 20 isconducting, and the ions which come into proximity thereto are attractedto the photoconductive layer 20 and pass directly to ground. In theremaining (dark) areas of the grid 16 the photoconductive layer 20 isinsulating and the ions, after building up a small potential (from a fewto a few hundred volts) on the surface of the grid, pass through theopenings in the grid 16 to charge the underlying surface of theinsulating layer 24. This modulation of control ofthe flow of ions fromthe corona discharge electrode 32 to the record medium 22 by the grid 16is described in more detail in connection with FIG. 8 below.

FIGS. 2-6 are perspective views showing alternative grid constructionswhich are useful in the present invention. FIG. 2 is a perspective viewof the grid 16 of FIG. 1 showing the photoconductive insulating layer 20and the core or grid 18. The grid 18 may be formed by etching orelectroforming. FIG. 3 shows a grid 40 consisting of a photoconductiveinsulating layer 42 on an electrically conductive electrode grid whichconsists of a series of equi-spaced, parallel electrodes 44 connected toa common electrical line 43. FIG. 4 shows a grid 50 formed from aperforated metal plate which fonns the electrode core or grid 52. Thegrid 52 is completely covered with a layer 54 of photoconductiveinsulating material. FIG. 5 shows a photoconductive insulating layer 56on an electrically conductive electrode core or grid 57. The core orgrid 57 can be formed, for example, by electroforming in which a durablehigh quality stainless steel plate is covered with a photoresist,exposed to the desired pattern (to harden the exposed areas of thephotoresist the photoresist in the background being subsequently washedaway), etched to leave posts and then electroplated with, for example,nickel, copper, gold or silver. The plating is then peeled off of thesteel plate resulting in an excellent electrically conductive electrodefoil which forms the core or grid 57. FIG. 6 illustrates a grid 60 inwhich an electrically conductive grid 61 is provided with a single,narrow slot or opening 62 having tapered faces 64. The surfaces of thegrid 61 adjacent the opening 62 are covered with a layer 66 ofphotoconductive insulating material. The opening 62 is of the order of0.1 mm. wide. The grid 60 finds use in connection with scanningprocesses, as is more fully discussed below. FIGS. 2-6 show examples ofthe various shapes and constructions which the photoconductive grid ofthe present invention can take. The resolution of the ultimate imagedepends on the number of openings or holes in the grid per linear inch,hereinafter referred to as lines per inch. The openings or holes in thegrids of FIGS. 2- 5 are on the order of about 50 to 500 lines per inch.The process of the present invention has been found to operate very wellwith 150, 200 and 300 lines per inch grids. It is noted that inscanning, the number of holes per inch does not determine the resolutionbut the size of the hole or slot, electrically and optically, does. Theterm lines per inch is only justified for non-scanning operations. Inthe scanning case resolution is more nearly the reciprocal of the holediameter in lines per unit length.

FIG. 7 is an enlarged cross-sectional view through a photoconductivegrid 70 which is identical to the grid 16 of FIGS. 1 and 2 except forthe nature of the layer 72 of insulating material which covers theelectrode core or grid 76. The grids of FIGS. 1-6 are all shown as beingcompletely covered with a photoconductive insulating material. In FIG. 7only a part of the insulating layer 72 is photoconductive. Theinsulating layer 72 of the grid 70 of FIG. 7 consists of aphotoconductive insulating layer 74 on one half of the grid 70 and anonphotoconductive, preferably opaque, insulating layer 78 covering theother half of the grid 70. Alternatively, an opaque insulating coatingcan be coated over the photoconductor on one side of the grid. The twolayers 74 and 78 meet to provide the complete insulating layer 72. Ingeneral, the photoconductive insulating layer 74 faces both the coronadischarge and the light source during exposure and imagewise charging;however certain embodiments of the invention, to be discussed below,employ different arrangements.

FIG. 8 illustrates how the flow of ions is modulated by the imagewiseexposed image grid means or photoconductive grid. The ions are eitherattracted to the gridin the exposed areas thereof (and thus removed fromthe flow of ions) or repelled from the grid due to the surface chargethereon in the unexposed areas (and thus pass through the openings inthe grid). For the purpose of this description, a grid 80 (similar,

for example, to the grid 16 of FIGS. 1 and 2) is shown having a groundedelectrode core or grid 82 completely covered with a layer 84 ofphotoconductive insulating material. The grid 80 is imagewise exposed asindicated by the small arrows 96. Ions, illustrated by the long, openarrows 86, are directed from a corona discharge (not shown) to the fieldelectrode 88 positioned behind the insulating record sheet 90, andthrough the grid 80. In striking the photoconductive layer 84 in theunexposed areas 92 thereof, the ions produce a small surface chargethereon. This charge will build up to somewhere between a few volts anda few hundred volts which will prevent further charging thereof andwhich will force the ions which would otherwise hit the unexposed areasof the grid 80 to flow around the grid structure and through theopenings in the grid 80. These ions, along with other ions which areflowing through the openings, continue through the grid and impinge uponthe insulating record sheet 90 to deposit charges thereon in theunderlying areas 94. However, in the imagewise exposed areas 93 of thegrid 80, the photoconductor becomes electrically conductive and any ionsstriking it pass through the photoconductive layer 84 to the electrode82 and to ground and are thus removed from the flow of ions.Furthermore, .the conducting areas 93 of the photoconductive layer 84have a trapping action extending a certain distancefrom thephotoconductive layer 84. As soon as the photoconductivity reaches acertain value, the distance at which the trapping action is effectiveextends to and beyond the middle of the openings or holes between theindividual elements or wires of the grid 80 and thus any ions 86directed toward such areas of the grid 80 are essentially completelytrapped; that is, the ions are drawn over to the photoconductive layer84 and pass to ground so that no ions pass through the exposed areas ofthe grid 80 to charge the insulating record sheet 90. There is, however,some leakage (pass-through of ions) in the exposed areas or light iswasted. This trapping action is substantially the same whether theinsulating layer surrounding the electrode grid 82 is allphotoconductiveor partly photoconductive and partly nonphotoconductive as shown in FIG.7. The degree of trapping depends not only on the amount of exposure andthe degree of photoconductivity of the photoconductor, but also on thepotential of the electrode grid 82 relative to that of the fieldelectrode 88.

If the electrode grid 82 has a potential somewhere between that of thecorona source and that of the field electrode 88, the trapping action inthe illuminated areas is somewhat reduced. If the grid 82 has apotential opposite in sign to that of the corona electrode the trappingaction in the illuminated areas is somewhat increased thus tending toclean-up the background and effectively increase sensitivity. It shouldbe noted that it is the complete coating of all the conductive surfaceof the grid within the picture area that allows the use of the preferredpotential opposite in sign to that of the corona electrode. Evenmicroscopic holes or cracks in the coating of photoconductor on the gridwill make the system wholly inoperative with the preferred bias.

It is noted that since the developing station is preferably remote fromthe charging station, developer powder or toner never needs to touch thephotoconductive grid; the photoconductive grid can be reusedindefinitely.

FIG. 9A illustrates an embodiment of the invention which employs acontrol grid means which can take the fonn of an additional electrodegrid between a photoconductive grid 102, which can be, for example, anyof those shown in FIGS. 27, and an insulating record sheet 104 carriedon a field electrode 106. The conductive grid 100 forms an electrostaticshield and does not absorb a majority of the ion flow. By this means thevoltage across the photoconductive grid 102 can be decoupled from thevoltage across the record sheet 104. The advantage of such decouplingwill be more fully understood by reference to FIG. 10 which is anapproximate equivalent circuit of the system shown in FIG. 1. A currentsource supplies current to a current divider circuit consisting of twobranches. One branch represents the photoconductive grid 16 of FIG. 1and is shown in FIG. 10 by a capacitor 112 in parallel with a variableresistor 116 (representing photoconductance). The other branchrepresents the air resistance between the grid 16 and the record medium22 of FIG. 1 and is shown in FIG. 10 by a resistor 118 in series with acapacitor 114 (the series capacitance of the paper). Since no currentflows from the grid 16 to the record medium 22, or vice versa, when thecurrent source (corona) is shut off, a diode is included in each branch.The transient behavior of this circuit can be analyzed in detail.However, the most significant characteristics involve the terminalvoltages on the capacitors. When the system is at equilibrium, there isno current flowing through the capacitor 114, so that it is charged tothe potential across the photoconductor, represented by variableresistor 116. All the current is flowing through the photoconductor 20so the final potential across it and the record medium 22 is equal tothe current through it times the photoconductor resistance. The recordmedium 22 potential is then limited in the simple grid system of FIG. 1to the current times the change in photoconductor resistance. It shouldbe noted that this equivalent circuit implies that the potentialdeposited on the record medium is limited to the maximum potential thatthe photoconductor can stand. This establishes a minimum thickness andresistivity in the dark of a given photoconductor in order to provide adevelopable image for any given development process. It should be notedthat at maximum deposited charge on the record medium the total flow ofcurrent is through the photoconductor while it has the maximum potentialacross it, resulting in maximum power dissipation in the photoconductor.

An approximate incremental equivalent circuit for the circuit of FIG. 9Ais shown in FIG. 11. FIG. 11 shows a current source 120 (the coronasource of FIG. 9A) which supplies current to two branches of a circuit.One branch represents the photoconductive grid 102 of FIG. 9A and isshown by a resistor 126 in parallel with a capacitor 122. The otherbranch represents the air resistance between the two grids and is shownby a resistor 119. Since there is no capacitor in series with theresistor 119, at equilibrium current will flow through the resistor 119.A dependent current source 121 supplies current to the paper capacitance123. The current source 121 supplies 'a current equal to or slightlyless than that which flows through the electrical resistance of the air.This means that in the steady state the current to the paper isindependent of the charge on it and that the paper can be charged to anarbitrary level by extending the charging time. In practice thephotoconductive grid 102 delivers its output into the additional grid100, which grid 100 appears to the grid 102 to be a grounded metalplate, though it is not actually grounded. The major part of the currentwhich is delivered to the grid 100, however, is transmitted through itto the record sheet 104 provided there is a potential of about 300 voltsor more (for convenient dimensions) between the record sheet 104 and theadditional grid 100 to accelerate the flow of ions. Since the flow ofions is independent of voltages above about 300 volts (for convenientdimensions) it-is possible to put a large potential between theadditional grid 100 and the record sheet 104.

This potential decreases the transit time of the ions between theadditional grid 100 and the record sheet 104 to the point wherediffusion of the image due to kinetic motion is negligible. The mainsource of diffusion is between the photoconductive grid 102 and theadditional grid 100. However, this is between two permanent parts of theapparatus which are stationary relative to each other and the amount ofdiffusion can be minimized by proper manufacture. Since the transit timeis inversely proportioned to the potential, the record sheet 104 can bepositioned more distant from the grid 100 by just increasing thepotential. In embodiments which do not employ the additional grid 100,ions diffuse at angles of about 45 so that the record sheet 104 shouldbe in virtual contact with the photoconductive grid 102.

FIGS. 12A and 12B give an idea of the limits of the applicability of theincremental circuit model of the double grid. A metal grid system wasset up as shown schematically in FIG. 12A. The current to a metalreceiving sheet 132 was measured as a function of the voltages on thetwo metal grids 130 and 131. In the area where the grid 131 -toreceivingsheet 132 voltage (V is above about 300 volts, the delivered currentdepends only on the intergrid voltage V,, and can vary between 0 andabout 4 microamps for the system shown. In this operating area thecharging of the paper is dependent on the charging time and theintergrid surface voltage V, only, making it possible to charge thepaper to hundreds of volts (limited only by breakdown or dischargethrough the insulator coating on the paper) with an intergrid potentialof a few volts.

FIG. 128 shows the characteristic output current of the circuit shown inFIG. 9A as a function of the additional grid 100 to record sheet 104voltage. The parameter of variation is the potential across thephotoconductive grid 102. These curves were actually obtained from theall-metal circuit shown in FIG. 12A

(using a 10 KV corona), in order to be able to measure the surfacepotential on each grid. Note that about 30 volts is all that is requiredto control a microampere delivered to 1,000 volts. This is approximatelythe charging current used to charge in 0.1 second over the 3 squareinches of the electrode used. The voltage gain is approximately a factorof 10, and is equal to the power gain of the device. In the embodimentshown in FIG. 1, in order to produce an acceptable image, thephotoconductive grid has to stand off or hold without-discharging asurface potential of about 300 volts. However, using the embodimentshown in FIG. 9A, having a second, all-metal grid 100, only a tenth ofthe voltage is needed at about the same current as before. One-tenth asthick a layer of photoconductor can be used on the photoconductive grid102. The thickness of the photoconductor layer on the photoconductivegrid 102 is the primary limiting factor of the device; therefore, theresolution can be theoretically increased by about a factor of 10.

If a thinner layer of photoconductive material on the photoconductivegrid 102 is not desired, it is also possible to use, instead, aphotoconductor with a higher dark current than was previously possible.This is particularly useful in extending the response of the system intothe infrared, where most of the photoconductors are characterized byhigh dark currents.

By attaching the two grids and 102 to each other through an occasionalintermediary insulating spacer, there are some advantages for largestationary exposures. The definition is no longer strongly dependent onthe spacing between the metal grid 100 and the record sheet 104 so thatsome bowing of the assembly due to gravity and/or electrostatic forcesis allowable. The field between the field electrode 106 and the metalgrid 100 exerts a force attracting the grid assembly to the record sheet104. Due to the incremental current source characteristic of the device,this field can be increased to a high enough level to overcome theattractive force of the corona source 108 on the photoconductive grid102. Thus, the arrangement shown in FIG. 9A, when using such spacers, issuitable for large spans without rigid support. The grid 100 ispreferably metallic and will wear well even if it touches the dielectricsurface of the record sheet 104. However, at these points, somecontact-charged spots will occur in the image and some contour to thesurface of the dielectric layer may be needed to minimize the areas ofthese spots.

The divorcing of the charging rate from the potential on the recordsheet 104 causes an increase in the average charging rate, i.e., thecharging proceeds at a uniform relatively high rate instead of taperingoff. Thus, the additional grid 100 effectively increases the currentgain of the system. Any difficulty in employing papers and developersthat would work well at low potentials can be eliminated by using theembodiment shown in FIG. 9A. It is possible to develop an insulatingrecord sheet on a temporary conducting backing, such as an insulatingpaper on a metal sheet, and then remove the record sheet from thebacking when the image is developed and fixed.

When using the embodiment of FIG. 9A with a scanning exposure step, acertain amount of difficulty is encountered. Normally, the systemappears in either the one or the two grid versions (FIG. 1 and FIG. 9A,respectively), to have a rapid decrease in response between 10 and 100cycles/second when the current is delivered to a metal plate. In the onegrid system (FIG. 1 the interaction between the charge on the recordsheet and the rate of current delivery acts like a negative feedbackloop and extends the frequency response at the expense of gain,producing acceptable scanned images. There is no evidence of such anoccurrence in the two-grid system (FIG. 9A). The primary use of theembodiment shown in FIG. 9A is for relatively high resolution and highsensitivity stationary exposure of images. A typical use would be inmaking X-ray photographs or electrographs. In such cases some additionalsensitivity may be secured by coating the metal grid 100 with an X-rayfluorescent electrically conducting coating. If the dimensions of thegrids 100 and 102 are appropriate, they may be held together by asimple, tacky adhesive layer, preferably with an equally perforatedinterleaving material therebetween to hold the two grids a certaindistance apart (usually about twice the distance between the openings inthe grid). Moire patterns can then become a problem, but such problemsare solvable by the methods used in color half-tone patterns. In thevisible spectrum, the two-grid device FIG. 9A is particularly adapted tostationary exposures for moderate-to-high resolution such as is requiredin a microfilm reader-printer or a hand-held camera.

The two-grid system of FIG. 9A can use any photoconductor that thesingle grid system of FIG. 1 can use. It is also possible to accomplishsome compensation for the electrical properties of some photoconductorsthat is impossible to do in the single-grid system of FIG. 1,specifically, there are some photoconductors that have sufficientresistance even when well-exposed to develop a sufficient surface chargeto allow some current to reach the paper, but when given an attractivebias, will attract the current and produce a clean image. Additionalbias can compensate for resistance in the exposed areas of thephotoconductors, rendering a usable image when at lower light levels,thus increasing the effective sensitivity of the system. Alternatively,photoconductors with very high impedance can be used if sufficient biasis used.

FIG. 9B shows an alternative arrangement of a twogrid system having thesame electrical characteristics as the arrangement shown in FIG. 9A, butwith improved image resolution. A foraminous insulating spacer 107 iscoated on its two surfaces with metal electrodes 101 and 103, which thusform metal grids corresponding to grid 100 and the metal core ofphotoconductor coated grid 102 of FIG. 9A. It is necessary in applyingmetal electrodes 101 and 103 to ensure that the metal does not coat theinner walls of the foraminous insulating spacer 107. The spacer 107 maybe made by drilling a regular array of small holes, typically 0.003inches in diameter, on 0.005 inch centers, in a sheet of insulatingplastic 0.006 inches thick. The thickness of the spacer should optimallybe about twice the diameter of the holes. The number of holes per linearinch determines the resolution of the finished print, and the individualholes should have a diameter as large as is mechanically consistent withthe center-to-center hole spacing. Metal electrode 103 is then coatedcompletely with photoconductor 105, either by evaporation or spraying,taking care that the holes are not filled, but that the edges ofelectrode 103 are thoroughly covered. The use of the spacer 107 preventsany migration of charge from one hole to another in the low field regionbetween electrodes 101 and 103 and thus improves the resolution of thefinished print. There is no significant sideways migration of charge inthe space between electrode 101 and receiving sheet 104 because of thehigh field in this region.

FIG. 13 shows a 3-grid system which provides a means for correctingfrequency response at the expense of current gain and resolution. FIG.13 shows a corona discharge electrode 140, an image grid means or aphotoconductive grid 142, an insulating record sheet 144 on a grounded,conductive field electrode 146 and a metal grid 148 analogous to themetal grid in FIG. 9A. The difference between the embodiment of FIG. 13and that shown in FIG. 9A is the use of another metal grid 150, placedin front of the 2-grid system of FIG. 9A, the photoconductive grid 142being closer to the rear grid 148 then to the front grid 150 and thegrids l48and 150 comprising the control grid means. The front grid 150is provided with a slight repelling bias and acts to limit the currentthrough the system. With a fixed bias on grid 148, the bias on grid 150is set at such a value that at an input frequency of I00 cps a smallchange in bias in one direction will not affect the current while asmall change in the other direction will affect the current. At lowerfrequencies this is also the maximum current that can flow, while athigher frequencies a smaller current will flow for the same inputamplitude.

In addition to the improvement in frequency response obtained with thearrangement shown in FIG. 13, this arrangement also has the advantage oflimiting the excessive buildup of charge on the photoconductive grid142. Referring to this advantage, the grid 150 can be referred to as alimiter grid. For its use as a limiter grid it can be much coarser (beof larger mesh) and have a spacing from the photoconductive grid 142which is large compared with the spacing of the latter from the recordsheet 144 or from the screen grid" when the limiter grid is used inconjunction with a double-grid system. The limiter grid therefore doesnot interfere appreciably with the optical image falling on thephotoconductive grid 142, and does not require a high degree ofmechanical precision in its construction. The purposes of the limitergrid are: (l) to limit the potential on the surface of the unilluminatedphotoconductive grid 142 and to therefore to prevent damage to thephotoconductor which might result from exceeding the voltage toleranceof the photoconductor, and (2) to prevent bulging or arching of thecenter of the photoconductive grid by shielding it from the strongelectrostatic field which is generated by the high-voltage corona wire.

The limiter grid is held at a potential, relative to the conductive coreof the photoconductive grid 142, which is of the same polarity as thepotential on the corona wire and which is of a magnitude roughly equalto the voltage which the photoconductor can withstand. As the surface ofthe photoconductor builds up potential to approach that of the limitergrid, the electrostatic field between the photoconductor surface and thelimiter grid is such as to prevent further charging of thephotoconductor. The potential across the photoconductor is thereforeheld to a safe value. The limiter grid can be used whether thephotoconductive grid is followed directly by the record sheet or byother gn'ds.

In some of the other embodiments of the subject invention the highvoltage impressed between the photoconductive grid and the corona wireproduces a strong electrostatic attraction which tends to bulge or archthe center of the photoconductive gn'd relative to its supports. Thischanges the spacing between the photoconductive grid and the recordsheet or the fol-

1. An electrographic recording apparatus for producing, on a layer ofinsulating material in contact with an electrically conductive backingmember, an electrostatic charge image corresponding to an image to berecorded, comprising: image grid means having electrically conductiveand electrically insulating areas for defining said image to berecorded; control grid means arranged in spaced and generally parallelrelation to said image grid means and to said backing member; means forindividually biasing each of said image grid means, control grid meansand backing member to a potential for establishing electrical fields ofdifferent strength between each of said areas, respectively, and saidbacking member; and means for directing a flow of ions toward said imagegrid means, control grid means and layer of insulating material, wherebysaid electrical fields modulate said flow of ions through said image andcontrol grid means to produce said electrostatic charge image on saidlayer of insulating material.
 2. The apparatus in accordance with claim1 wherein said image grid means comprises at least one electricallyconductive core completely coated with a layer of electricallyinsulating material at least a part of which is responsive to imagewiseexposure to radiant energy to form said conductive and insulating areas.3. The apparatus in accordance with claim 2 wherein saidradiant-energy-responsive part of said insulating material isphotoconductive.
 4. The apparatus in accordance with claim 1 whereinsaid image grid means comprises at least one electrically conductivecore completely coated with a photoconductive insulating materialresponsive to imagewise exposure to radiant energy to form saidconductive and insulating areas.
 5. The apparatus in accordance withclaim 1 wherein said control grid means comprises at least oneelectrically conductive control grid arranged in spaced and generallyparallel relation to one side of said image grid means.
 6. The apparatusin accordance with claim 5 wherein said one control grid is arrangedbetween said image grid means and said backing member.
 7. The apparatusin accordance with claim 1 wherein said control grid means comprises anelectrically conductive grid Arranged to that side of said image gridmeans that is opposite from said backing member.
 8. The apparatus inaccordance with claim 1 wherein said control grid means comprises a pairof electrically conductive grids, one of which is located between oneside of said image grid means and said insulating surface and the otherof which is located to the opposite side of said image grid means. 9.The apparatus in accordance with claim 1 wherein said directing meanscomprises a corona discharge device.
 10. The apparatus in accordancewith claim 1 wherein the polarity of the potential applied to said imageand control grid means and said backing member by said biasing means isthe same as that of the ions in said flow of ions.
 11. An electrographicrecording apparatus for producing, on a layer of insulating material incontact with an electrically conductive backing member, an electrostaticcharge image corresponding to an image to be recorded, comprising: animage grid comprising an electrically conductive core sequentiallyconnectable to sources of different potential relative to said backingmember and completely covered with a coating of electrically insulatingmaterial that is a photoconductive insulating material on at least oneside of said core, said grid being arranged in spaced and generallyparallel relation to said backing member; a control grid that iselectrically conductive, sequentially connectable to sources ofdifferent potential relative to said backing member and arranged inspaced and generally parallel relation to said image grid on the sidethereof that is opposite said backing member; means for generating ionsto provide a first and a second flow of ions toward said control andimage grids; means for connecting each of said core and control grids toone of its respective sources of potential, while said first flow ofions is being generated, whereby said photoconductive material will beuniformly charged to a potential generally equivalent to that on saidcontrol grid; means for imagewise exposing said photoconductiveinsulating material, after said image grid has been uniformly charged,to create electrically conductive and insulating areas thereoncorresponding to said image to be recorded, and means for connectingeach of said core and control grid to another of its respective sourcesof potential, while said second flow of ions is being generated forestablishing electrical fields of different strength between each ofsaid areas respectively and said backing member, whereby said fieldswill modulate said second flow of ions through said image and controlgrids to produce said electrostatic image on said layer of insulatingmaterial.
 12. The apparatus in accordance with claim 11 wherein, duringsaid first flow of ions, the potential applied to said image grid bysaid connecting means is of opposite polarity to said ions, and thepotential applied to said control grid is substantially equal to that ofsaid backing member.
 13. The apparatus in accordance with claim 11wherein, during said second flow of ions, the potential appliedrespectively to said image and control grids by said connecting means isof the same polarity as said ions.